1. Field of the Invention
This invention relates to a friction material comprising carbon fiber reinforced carbon (hereinafter referred to as "CFRC") which has an oxidation temperature of at least about 800.degree. C. and a crystallinity index R of the matrix of from about 2.3-5.0, a process for the production thereof, and friction elements using said CFRC.
2. Description of the Prior Art
CFRC has increasingly been attracting attention as a material for use in various friction materials, particularly in brakes of aircrafts, because of its light weight and good friction characteristics. For the production of such CFRC friction material, the following methods are known:
(a) A carbon fiber is impregnated with a carbonizable resin, molded and carbonized, and the procedure of impregnation and carbonization is repeated to increase the density of the CFRC;
(b) Pyrolysis carbon is deposited on a carbon fiber by a chemical vapor deposition method (cf. SAMPE 16th National Symposium and Exhibition p 257; April, 1971) and
(c) A carbon fiber is impregnated with a carbonizable resin, molded and carbonized and thereafter the density of the CFRC is increased by a chemical vapor deposition method.
With conventional friction materials, particularly aircraft brake materials comprising CFRC, it has been considered to be preferred that while conducting the steps of method (a), (b) or (c), or between the two steps in said methods, or after the completion of the steps thereof, the CFRC is subjected to at least one heat-treatment at a high temperature, generally a temperature of at least 2,500.degree. C., to effect the graphitization of matrix carbon; see U.S. Pat. No. 3,970,174 (corresponding to Japanese Patent Application (OPI) No. 101770/1975). This is because high graphitization of carbon leads to a reduction in abrasion loss during braking, which is desirable for a brake material.
Such graphitization, however, gives rise to the problems that production costs are increased since the heat-treatment of CFRC at the high temperature as described above is required for the graphitization and CFRC is softened, resulting in a decrease in strength and stiffness. However, conventionally if the CFRC is not graphitized, the abrasion loss of the brake material during the braking increases, which is not desirable for the brake material.